Several techniques have been developed for noise reduction. These include, for instance, the use of passive mufflers, such as those found on the exhaust systems of automobiles. Other techniques include the use of noise-reducing enclosures around the noise-creating device and sound-absorbing materials to reduce the reverberation of sound in the environment. In addition, active techniques using the generation of "counternoise" to neutralize the noise have also been demonstrated successfully. For example, a system of electrically powered microphones for detecting noise, linked to electrically powered speakers for generating a counternoise, has been used successfully in the cabin of propeller-driven aircraft. The electrical microphone-speaker system requires a plurality of these devices distributed along the walls of the cabin, and is limited to reducing noise within a narrow bandwidth. Thus, the system is well adapted for attenuating the periodic sound pressure generated by a rotating impeller, but is not well suited for reducing the broad sound wave band generated by a jet engine or the aerodynamic boundary layer of a flying aircraft.
There exists a need for a device that is able to attenuate sound waves, across a broad frequency band, that is reliable and cost-effective. Preferably, the device should not require significant input of maintenance, and should be able to operate effectively for long periods of time without continuous monitoring. Furthermore, the device should desirably be energy efficient, either not using power, or using very little power. Moreover, the device should be space-efficient, and not bulky, so that it can be readily used in a variety of applications where space limitations are important. Finally, the device should also be light weight to allow use in weight-sensitive applications, such as aircraft cabins.